The capacity and pressure requirements of a system can be shown with the use of a graph called a system, curve. Similarly, a capacity versus pressure variation graph can be used to show a given pump's performance. As used herein, “capacity” means the flow rate with which fluid is moved or pushed by a pump, which is measured in units of volume per unit time, e.g., gallons per minute. The term “pressure” relative to fluids generally means the force per unit area that a fluid exerts on its surroundings. Pressure can depend on flow and other factors such as compressibility of the fluid and external forces. When the fluid is not in motion, that is, not being pumped or otherwise pushed or moved, the pressure is referred to as static pressure. If the fluid is in motion, the pressure that it exerts on its surroundings is referred to as dynamic pressure, which depends on the motion.
The variety of conditions, ranges, and fluids for which it can be desirable to measure pressure has given rise to numerous types of pressure sensors or transducers, such as but not limited to gage sensors, vacuum sensors, differential pressure sensors, absolute pressure sensors, barometric sensors, piezoelectric pressure sensors, variable-impedance transducers, and resistive pressure sensors. One problem with the use of pressure transducers is that, depending on the composition and materials used in the transducer and the composition of the fluid being measured, the transducer can break down and contaminate the system. Another problem with the use of pressure transducers is that their accuracy can vary both with temperature changes and over time. Temperature changes and large pressure changes typically occur during semiconductor wafer processing with supercritical fluids. During wafer processing, the unreliable accuracy of pressure sensors can adversely impact quality control and affect yield. It would be advantageous to have a fluid flow control system that does not include pressure transducers. It would be desirable to eliminate the need for using pressure transducers in controlling the flow of a fluid during semiconductor wafer processing.
Flow meters are commonly used to measure a fluid flow in the processing of semiconductor wafers and other objects. Problems commonly associated with flow meters include clogging, contamination, leaks, and maintenance costs. It would be advantageous to have a fluid flow control system that does not include flow meters. It would be desirable to reduce contamination in semiconductor wafer processing by elimination of the contamination typically associated with the use of flow meters.
The use of pumps in the processing of semiconductor wafers and other objects is known. Pumps induce fluid flow. The term “head” is commonly used to measure the kinetic energy produced by a pump. By convention, head refers to the static pressure produced by the weight of a vertical column of fluid above the point at which the pressure is being described-this column's height is called the static head and is expressed in terms of length, e.g., feet, of liquid.
“Head” is not equivalent to the “pressure.” Pressure has units of force per unit area, e.g., pound per square inch, whereas head has units of length or feet. Head is used instead of pressure to measure the energy of a pump because, while the pressure of a pump will change if the specific gravity (weight) of the fluid changes, the head will not change. Since it can be desirable to pump different fluids, with different specific gravities, it is simpler to discuss the head developed by the pump, as opposed to pressure, neglecting the issue of the specific gravity of the fluid. It would be desirable to have a fluid flow control system that includes a pump.
There are numerous considerations and design criteria for pump systems. Pump performance curves have been used as tools in the design and analysis of pump systems. FIG. 1 is a representative illustration of a pump performance curve for a centrifugal pump with various impeller diameters, for the purpose of showing the relationship between the capacity (flow rate) and total dynamic head of an exemplary pump in the prior art. As a general rule with centrifugal pumps, an increase in flow causes a decrease in head. Typically, a pump performance curve also shows the rotational speed in revolutions per minute, net positive suction head (NPSH) required, which is the amount of NPSH the pump requires to avoid cavitation, power requirements, and other information such as pump type, pump size, and impeller size. For example, the pump size, 1½×3-6, shown in the upper part of the centrifugal pump curve illustrated in FIG. 1, indicates a 1½ inch discharge port, a 3 inch suction port, and a maximum nominal impeller size of 6 inches. As depicted in FIG. 1, the several curves that slope generally downward from left to right across the graph show the actual performance of the pump at various impeller diameters. Pump system performance can vary for every application based on the slope of the pump performance curve and its relationship with any specific system curve.
What is needed is an apparatus for and method of controlling a fluid flow for use in the processing of an object with a fluid, such that contaminants in the fluid are minimized. What is needed is an apparatus for and method of controlling a fluid flow that does not include flow meters for controlling the fluid flow. What is needed is an apparatus for and method of controlling a fluid flow that does not include pressure transducers for controlling the fluid flow.